The model your child drew is correct and almost there, just needed a few additional steps!

Divide Yvonne’s units into 6 parts (to make it divisible by 3 and 2). This means Xavier has 3 units (half of Yvonne’s) and Zach has 2 units (1/3 of Yvonne’s). Since Xavier has 19 more stickers than Zach, this means: